Long arc column forming plasma generator

ABSTRACT

In an improved long arc column forming plasma generator an aerodynamic nozzle face configuration and a novel two part insulated housing cooperate to substantially reduce so-called &#39;&#39;&#39;&#39;double arcing&#39;&#39;&#39;&#39; of the plasma column and damage to the plasma generator caused therefrom.

United States Patent 1191 Camacho June 18, 1974 [54] LONG ARC COLUMNFORMING PLASMA 3,194,941 7/1965 Baird "I. 219/121/1 x GENERATOR3,221,212 11/1965 Gorowitz eta. 315 111 1 3,242,305 3/1966 Kane et al219/75 Inventor: Salvador Camacho, ig C 3,294,953 12/1966 Spies, Jr.219/121 P 3,375,392 3/1968 Brzozowski et al 313/231 [73] Assgnee g i'q3,588,594 6/1971 Yamamoto et a1.. 219/121 P x Cowman, a 61gb 3,674,9787/1972 Becker et al. 219/121 P [22] Filed: Nov. 9, 1972 PrimaryExaminerJ. V. Truhe [21] Appl 305092 Assistant Examiner-G. R. Peterson[52] US. Cl. 219/121 P, 219/75, 313/231 57 ABSTRACT [51] Int. Cl ..B23k9/00 1 58 Field 61 Search 219/121 P, 74, 75; In an Improved long areColumn forming Plasma 313/231 1 1 13/9, 31 erator an aerodynamic nozzleface configuration and a novel two part insulated housing cooperate tosubstan- [56] References Cited tially reduce so-called double arcing ofthe plasma column and damage to the plasma generator caused UNITEDSTATES PATENTS therefrom 3,097,292 7/1963 Kugler et al 2l9/12l P3,130,292, 4/1964 Gage et a1 219/75 7 Claims, 6 Drawing Figures I. I I.I. I. I. I. III-1 LONG ARC COLUMN FORMING PLASMA GENERATOR BACKGROUND OFTHE INVENTION 1. Field of the Invention This invention relates to plasmaare devices and methods and particularly to those devices and methodsadapted for double arc abatement.

2. Description of the Prior Art Methods and apparatus for generating along (i.e., 12-40 inches) plasma arc column have been known. In what isperhaps the closest known prior art there is disclosed in U.S. Pat. No.3,673,375 a long are column forming plasma generator. In this patentrecognition is given to the importance of the relation between nozzleinternal diameter, nozzle length, and vortex chamber width in obtaininga long directionally stable plasma arc column greater that 12 inches,while minimizing internal electrode deterioration. While this disclosureconstitutes a significant advance in the art a need for furtherimprovement has been revealed.

Such long arc column forming plasma generators as described in the abovecited reference and elsewhere in the prior art, have characteristicallyincluded a cylindrical internal electrode forming a gas vortex chamberwithin itself, a gas directing nozzle located forwardly of and insulatedfrom the cylindrical electrode, and a cylindrical outer housing securedto the nozzle and enclosing the electrode. That portion of thecylindrical outer housing residing in proximity to the nozzle has in thepast been electrically connected to the nozzle and susceptible to damagefrom so-called double arcing of the plasma column.

Double arcing occurs in one case when one arc is formed inside thegenerator between the internal electrode and the nozzle, and another areis formed outside the generator between the nozzle and a workpiece alsoin the electrical circuit. In another case, double arcing may occur, forexample, during scrap steel melting when a transferred plasma arc isformed between the internal electrode and a metal scrap and anotherreturn are is formed between another portion of the scrap and thegenerator housing. In either case, substantial damage to the nozzle,housing or both nearly always results. Although methods and apparatusfor abating double arcing have been attempted in the past, none haveproven commerically successful.

Furthermore, a long are column forming plasma generator of the classdescribed has heretofore often been mounted in a furnace or the like bymeans of an insu lated collar to prevent grounding of the generatorhousing against the furnace mountings. Such grounding shorts theelectrical circuit preventing operation in most installations and at thesame time poses a severe shock hazard to personnel in the area of themounting apparatus. U.S. Pat. No. 3,689,732 is broadly related to thisinvention in its teaching of a sleeve formed from an electricalinsulator which is adapted for use on a hand held electric arc device.Such a sleeve, however, would be wholly unsuited to the needs of thepresent invention with respect to double arc abatement, due to itsinability to withstand the extremely high temperatures of a plasma arccolumn without rapidly deteriorating. It would, therefore, serve nopractical purpose for double arc abatement on such a plasma generator.

Based on the above, there is a need for an improved long arc columnforming plasma generator which is no longer susceptible to damage fromdouble arcing, which is capable of being grounded directly to a mountingdevice thereby eliminating the need for an insulated mounting collar oflining, and which at the same time is able to effectively circumvent thepossiblity of severe electrical shock to personnel in the area of agenerator mounting device.

SUMMARY OF THE INVENTION The method and apparatus of the presentinvention are directed to an improved long arc column forming plasmagenerator based on the long are column forming plasma generator taughtin the above cited U.S. Pat. No. 3,673,375. An improved long are columnforming plasma generator according to the invention is primarilydirected at the abatement of double arcing of the plasma column andcomprises a two part cylindrical insulated and water cooled housing andan improved nozzle face configuration. One member of the two parthousing is adapted to overlay the rearward or mounting end of thegenerator, while a second housing member is adapted to overlay theforward or nozzle end of the generator. Once installed, the respectivehousing members are adapted to be electrically insulated from each otheras well as from the generator nozzle and internal generator parts. Theresult is that the plasma column is in no way attracted to the housingmember overlaying the nozzle because of its isolation from theelectrical circuit. Double arcing to the housing is thus effectivelyprevented. Moreover, in order for the arc column to double arc to therear housing member or that housing in closest proximity to the mountingend of the generator, it must traverse the forward housing member byforming two external arcs simultaneously. The likelihood of thisoccurrence is very small. In addition, water cooling of the housingmembers further limits the possibility of a double arcing situationsince an arc column will be attracted to hotter rather than coolersurfaces as is well-known. Finally, this invention contemplates the useof an aerodynamic nozzle face configuration which is adapted to cause anon-laminar flow of air across the nozzle face during operation of thearc column. The result is a greatly stabilized arc column andsubstantial reduction in double arcing and resulting damage to thenozzle.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cross sectional view ofa long arc column forming plasma generator based on those of the priorart, which has been modified in accordance with the invention.

FIGS. 2 and 3 show cases of the prior art in which destructive doublearcing has been known to occur.

FIG. 4 shows a prior art plasma generator in a typical insulatedmounting in which the accumulation of kish has caused short circuitingacross a prior art insulated mounting collar resulting in damage to theinsulator as well as the generator.

FIG. 5 is a detailed broken view of a plasma generator of the preferredembodiment.

FIG. 6 is a somewhat enlarged cross sectional view showing an insulatedjoint employing Fibrefrax insulation according to the preferredembodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, animproved long arc column forming plasma generator constructed accordingto one embodiment of the present invention is generally designated 10.Such improved plasma generator is herein shown in simplified form toeliminate unnecessary and well-known internal construction details. Theparticular embodiment shown in FIG. 1 represents a plasma generator ofthe prior art which has been adapted to receive a two-part cylindricalhousing 12 according to this invention. Generally, such a plasmagenerator of the prior art has included a cylindrical internal electrode13 and a gas directing nozzle 14 forwardly spaced and insulated fromsaid electrode 13. Other conventional parts are included such as gas,water and electrical supplies. Known long are column forming plasmagenerators of the prior art have also included a one-piece cylindricalhousing (not shown) secured to the nozzle 14. Reference is made by wayof example to previously cited U.S. Pat. No. 3,673,375 issued to thesame inventor, and specifically to FIG. 1 of that patent which shows thedescribed unitary housing. Such prior art housings have been in physicalas well as electrical attachment with the nozzle and as previouslymentioned any current above ground which the nozzle received by way ofadouble arcing condition, later described in detail, was also passedthrough the entire housing. Referring still to FIG. ll this inventionprovides a housing comprised of at least two separated and insulatedmembers l8, 19 instead of the prior art unitary housing in order toprevent such double are induced current from being connected with suchhousing. A first cylindrical housing member 18 is adapted to be securedvia an appropriate electrical insulator 24, to said nozzle 14.Securement is made by appropriate threaded or forced fit connection suchthat insulator 24 is effectively sandwiched between housing member 18and nozzle 14. A second cylindrical housing member 19 is adapted to belocated rearwardly of and slightly spaced from said first housing member18. Additional electrical insulating material is adapted to line thespace 26 formed at the joint 27 between housing members l8 and 19.Housing members 18 and 19 are preferably formed from stainless steel.Insulators 24 and 25 are preferably formed from Synthane insulatingmaterial and may assist in supporting housing members 18 and 19 inaddition to providing electrical insulation therebetween. Additionalinternal support of insulator 25 and housing members 18 and 19 may beprovided by radial support members 29. Finally, insulated separation ofhousing members'l8 and 19 from each other and from nozzle 14 enable therearward housing member 19 to be grounded directly to furnace mountingsor r the like.

Since the abatement of what has been termed double arcing" is theforemost object of this invention, attention shall now be directed to adiscussion of known instances of double arcing" in the prior art.Subsequently, the description will turn to a discussion of the preferredembodiment.

Referring now to FIG. 2 a long are column forming plasma generator ofthe prior art, generally designated 38 is shown in a sidewall mountedoperating position inside a conventional fumace 39 adapted for meltingscrap metals 35. Such a plasma generator as previously stated normallyincludes a one-piece cylindrical steel housing 37 pysically andelectrically connected to the nozzle 14 (shown in cutaway). Duringnormal operation of such a generator the transferred plasma arc willfollow an axis generally indicated by dashed line X. Quite often,however, the distance along axis X to a metal scrap may exceed the outerrange of the transferred column due to a pocket in the scrap or suddenmelting or shifting, for example. When this occurs the arc column isforced to bend slightly as indicated by dashed line Y in order to strikea metallic object within its range. In many cases further bending of thearc column will ensue, with the ultimate result of the creation of adouble arc. A double arc in this instance comprises an internal are 40extending between internal electrode and nozzle 14, and an external are41 extending between nozzle 14 and/or housing 38 and proximate scrapmetal 35. Once a double arc has been created external are 41 oftencreeps rearward to encounter additional proximate metallic scrap asindicated by dashed line 42. Extensive damage to the housing 37 and thenozzle 14 nearly always results.

Referring now to FIG. 3 which shows a second typical case of doublearcing," a plasma column being generated along a nonnal axis isindicated at Z. Due to electrical connection throughout the scrap 35,current may return back to the generator housing 37 or nozzle 14 byforming an external arc 43 between the housing 37 and a proximateportion of the scrap indicated at 36. Are 43 being of shorter lengththan the are shown at Z may cause the are at Z to be extinguished andsimultaneously therewith may cause an internal arc to form identicalwith 40 in FIG. 2. Again, substantial damage nearly always results.

Referring now to FIG. 4, a long are column forming plasma generator ofthe prior art is generally designated by numeral 41. Mounting of suchgenerator in a furnace 39 has in the past required the use of insulatedcollars 43, linings and the like in order to prevent grounding of thegenerator housing 37 against the furnace walls 39'. Such groundingprecludes normal operation of the arc circuit, in addition to creating asafety hazard to persons near the mountings. A stainless steel shell 45has normally housed such insulated collar and retained it within anappropriate mounting aperture 46. A usual problem of such collars 43when in use for prolonged periods of time is the gradual accumulation ofkish generally represented by 48. Kish is widely known as a particulatesubstance resembling graphite, comprising carbon, iron and manganese andwhich often becomes airborne during iron smelting operations. When theaccumulated kish 48 becomes sufficient to form an electricallyconductive pathway across insulated collar 43, an external arc will formbetween housing 37 and steel shell 45 and simultaneous therewith thetransferred are T will be extinguished and an internal arc formedsimilar to that of 40 in FIG. 2. Substantial damage to housing 37 andcollar 43 immediately results. A potentially fatal shock hazard alsoexists to those persons who may accidentally contact the generatorduring this event.

Continuing with the description referring again to FIG. 1, it isapparent that a long are column forming plasma generator utilizing a twopart housing constructed and insulated in accordance with the firstdescribed embodiment is capable of being mounted directly to a furnaceaperture without the usual insulated linings, because the rear housingportion of said generator is insulated at considerable distance (thelength of forward housing member 18) away from nozzle 14. In addition,housing member 18 being in effect electrically floating prevents currentoriginating in nozzle 14 from being conducted rearward to the generatormountings. Double arcing of the plasma column to the housing, damage andelectrical shock hazard caused therefrom are thus effectivelyeliminated.

While the invention embodiment described in connection with FIG. 1satisfies the foremost objects of the instant invention it has beenrecognized that such a plasma generator which employs Synthaneinsulation or the like to insulate the respective housing members 18 and19, is limited to low power and temperature applications due to thedegradation of Synthane insulation at extremely high temperatures.Furthermore, while double arcing of the plasma column is substantiallyprevented from contacting and damaging the housing, double arcing to thenozzle itself has not been abated by providing a two part insulatedhousing. Finally, this invention contemplates that a one-piece Ihousing, not shown, insulated only a short distance from nozzle 14 wouldbe substantially more susceptible to a double arc than would a two piecehousing in which two separated external arcs (See 18' and 19' in FIG. 1)would be required to form simultaneously to create a double arcingcondition. This invention has discovered that the probability of thelatter occuring can be reduced even further by water cooling therespective housing members and the probability of double arcing to thenozzle can be diminshed by inducing a non-laminar flow of air across thenozzle face by means of a novel nozzle face configuration.

Referring now to FIG. 5, the apparatus of the preferred embodiment isdirected to a long are column forming plasma generator as thatpreviously described in connection with FIG. 1. In the preferredembodiment a two-part housing comprises members 51 and 52. Each housingmember 51, 52 is adapted for cooling by internal manifold structures 60,61 which form passages 63 for the reception of coolant, e.g., water.Likewise, internal electrode 55 has been adapted by manifold structure64 for the reception of coolant as indicated at 65. Since the arcoriginates internally along a distance of internal electrode 55indicated by D it is desirable to forceably cool this region especiallywhenthe generator is being operated in extremely high powerapplications. Manifold 64 has therefore been adapted to form a narrowerpassageway nearest the region designated D of internal electrode 55 thanrearward of said region in order to accelerate the flow of coolant,e.g., water over the said critical region. Coolant is adapted to flowthrough manifold 64 in the directions indicated by arrows .68. Watercooling of the housing members 51, 52 has the effect of minimizing thepossibility of double arcing to the housing due to the reduced surfacetemperature. Water cooling of the housing members as well as theinternal electrode have the added advantage of prolonging the life ofthe parts in high temperature and power applications. While watercooling of the housing members 51, 52 as well as the internal electrode55 is regarded as novel in the instant invention, the importance ofwater cooling of the nozzle 56 has been known in the art for sometime.It satisfies the needs of this invention to state that nozzle 56 isadapted by internal passages, indicated by dashed lines 71, to

receive coolant through conduits 72. Further elaboration upon the nozzlecooling structure is not deemed necessary herein.

The face configuration of nozzle 56 has been adapted to include asomewhat annular shaped depression 66 formed near the center bore 69.Said depression has been adapted in the instant invention to prevent thelaminar flow of air across the heretofore relatively flat nozzle faceduring operation of the arc column. Such laminar flow of air isdiagramatically indicated by arrows 73, '74 in FIG. 1 wherein nozzle 14exhibits a face configuration typical of the prior art long arc columnforming plasma generators. During operation of the plasma column P, airhas been found to flow inward across the flat nozzle face and follow thecurvature into the bore 69. Such flow of air has been found tocontribute to internal arcing to the nozzle itself. This inventioneffectively breaks the laminar air flow by providing the undulations inthe nozzle face 76 of the preferred em bodiment created by annulardepression 66. Air now tends to circulate within the depression 66 asindicated by arrows 78. The drastic reduction of the flow of air intothe bore 69 appears to enhance vortex stability of the plasma columnsince internal arcing is noticeably reduced.

A further unique aspect of the invention relates to effectivelyinsulating the various cooperating parts of this invention, i.e.,housing members 51, 52 improved nozzle 56 and internal electrode 55 insuch a manner as to permit greatly prolonged use of the plasma generatorin extremely high temperature andpower applications and in the virtualabsence of destructive double arcing. In this regard as shown in FIGS. 5and 6, cylindrical insulator 81 is adapted to extend partially thelength of the plasma generator and is adapted to supportively linehousing members 51 and 52. A second cylindrical insulator 82 is adaptedto extend partially the length of the plasma generator to supportivelyline the outside of the combined internal electrode 55 and watermanifold structure 641. Insulator 82 is also adapted to line the spacesbetween nozzle 56 and internal electrode 55 which has in the past beenlined with an annular insulator (See 21 FIG. 1) to prevent electricalshort circuiting therebetween. Arc gas flow is adapted to bypassinsulator82 of the preferred embodiment and is appropriately ductedthrough tubes 85 and channels 86, indicated by dashed lines, provided innozzle 56. Insulators 81 and 82 are preferably formed from Synthanematerial and may be provided with threads 88, 89 for purposesofsecurement to nozzle 56. Instead of Synthane material which is availablefrom Synthane Corporation, 1 River Road, Oaks, Pa. there may beemployed, e.g., Deln'n Nylon, made by DuPont, ordinary Nylon or thewell-known Westinghouse Micarta type insulating materal. Problems havebeen experienced at high temperatures when the so-called Synthaneinsulation is exposed to heat. Accordingly, this invention contemplatesemploying Synthane or like forms of insulation in an enclosed manner bysealing exposed portions of insulator 81 with a so-called Fibrefraxrefractory insulation 96, FIG. 6. Such an insulating refractory isavailable from the Carborundum Company, Niagara Falls, NY, in amortar-like refractory compound which solidifies when heat treated,becomes highly reflective, and extremely high temperatures. So-calledRam-90 insulating refractory material made by Har- 7 binson-walkerCompany of Pittsburgh, Pa. may also be employed.

Curved recesses 91 are formed between the forward end of housing member51 and nozzle 56, best shown in FIG. 6. Curved recesses 92 are alsoformed between the opposite end of housing member 51 and the forward endof housing member 52. Although not shown on the drawing, housing member52 preferably includes an integral end plate at the rearward end whichconsequently does not require insulated sealing by the Fibrefraxinsulation. It is apparent that insulators 81 and 82 may be formed frommany interlocking cylindrical pieces, that nozzle face 76 may employvarious alternate configurations and that the invention housing maycomprise two or more housing members and still be consistent with theintended breadth and scope of this invention.

Referring specifically to FIG. 6, is should be noted that the concaverecess 91 formed in nozzle 56 resides opposite a convex tip portion 91'of forward housing section 51 and that the annular void between recess91 and tip portion 91 is filled with the insulating refractory material90. Such a disposition of mated and spaced curved surfaces between thenozzle and housing eliminates any sharp points of electrical fieldconcentrations on either nozzle or housing, makes the electrical fieldessentially uniform through the insulating refractory material 90 andthus avoids electrical breakdown between housing 51 and nozzle 56.Furthermore, the insulating refractory in this configuration isessentially locked into place once it has been heated. The curvedrecesses 92 act similarly.

With regard to the specific geometry of the nozzle, the teaching of thepreviously referred to U.S. Pat. No. 3,673,375 is adopted andincorporated herein by reference. The cross sections in particularshould be understood as representing annular or cylindrical shapes asthe case may be. Using FIG. 4 of that patent as a reference and thedesignation C for the nozzle internal diameter, the designation B forthe nozzle length and the designation A for the nozzle chamber width,the relationship B/C O.2 for the transferred mode and B/C 2 4 for thenon-transferred mode when used in the present invention offers both theadvantage of a long arc and freedom from double arcing. Since much ofthe structure employed in the present invention is shown in U.S. Pat.No. 3,673,375 such patent is also referred to for other mechanicaldetails which were deemed unnecessary to more fully explain in thisdescription.

What is claimed is:

I. In an elongated generally tubular apparatus adapted to generate along are high temperature plasma between its discharge end andelectrical conductor in an arc circuit which remains substantially freeof all but a main arc, in combination:

a. an external tubular thin wall housing fonned of a plurality ofaligned tubular sections of electrically conducting material including aforward section extending from the discharge end of the apparatusrearwardly, each section including said forward section beingelectrically insulated from the next rearward section and to the extentthat any section provides a point of double arc attachment each suchsection being further insulated from electrical 8 ground;

b. a fluid cooled cylindrical shaped electrode centrally positioned andsupported within said housing and having its forward end proximate thedischarge end of the apparatus and its rearward end positioned toreceive the usual plasma gas supply therein;

0. a fluid cooled gas directing nozzle axially aligned with, forwardlyspaced and insulated from both said electrode and housing forwardsection, said nozzle with said electrode providing a vortex formingchamber, said nozzle having an internal diameter designated C and alength designated B and with said electrode providing said vortexforming gas chamber of a width designated A, said dimension A beingselected as the minimum width at which a vortex strength of 0.25 Mach isobtained when B is of minimum arc sustaining width and C is equal to theinternal diameter of said electrode, and B a d C hav t e elat n-s iny?2.0- fe the transferred mode and B/C 2 4 for the nontransferred mode;

d. gas supply means for introducing an arc gas through said electrodeinto said chamber to produce a vortical flow in said chamber and nozzle;and

e. cooling fluid supply means for introducing a cooling fluid into saidelectrode and nozzle to cool the same.

2. An apparatus as claimed in claim 1, said nozzle having an annulardepression formed in the forward end of said nozzle and in a formeffective to prevent laminar flow in the space immediately adjacent theforward end of said nozzle and surrounding the plasma generated by saidapparatus.

3. An apparatus as claimed in claim 2 wherein selected said housingsections are adapted with cooling passages to be fluid cooled andincluding means to provide a cooling fluid to such passages.

4. An apparatus as claimed in claim 1 wherein the rearwardmost saidhousing section is electrically grounded.

5. An apparatus as claimed in claim 1 wherein selected said housingsections are adapted with cooling passages to be fluid cooled andincluding means to provide a cooling fluid to such passages.

6. An apparatus as claimed in claim 1 wherein said forward housingsection and nozzle are shaped to provide opposed spaced oppositelycurved surfaces between the forward inner end portion of said forwardhousing section and the oppositely disposed outer nozzle surface toestablish a substantially uniform electrical field between said surfacesand including an insulating refractory material filling the spacebetween said surfaces. 7

7.. An apparatus as claimed in claim 1 wherein said fluid cooledelectrode includes fluid cooling passages proximate the forward internalare generating surface of said electrode formed in a manner adapted toprovide a higher rateof heat removal from such surface than from themore rearward internal non-arc generating surface of said electrode.

1. In an elongated generally tubular apparatus adapted to generate along arc high temperature plasma between its discharge end andelectrical conductor in an arc circuit which remains substantially freeof all but a main arc, in combination: a. an external tubular thin wallhousing formed of a plurality of aligned tubular sections ofelectrically conducting material including a forward section extendingfrom the discharge end of the apparatus rearwardly, each sectionincluding said forward section being electrically insulated from thenext rearward section and to the extent that any section provides apoint of double arc attachment each such section being further insulatedfrom electrical ground; b. a fluid cooled cylindrical shaped electrodecentrally positioned and supported within said housing and having itsforward end proximate the discharge end of the apparatus and itsrearward end positioned to receive the usual plasma gas supply therein;c. a fluid cooled gas directing nozzle axially aligned with, forwardlyspaced and insulated from both said electrode and housing forwardsection, said nozzle with said electrode providing a vortex formingchamber, said nozzle having an internal diameter designated C and alength designated B and with said electrode providing said vortexforming gas chamber of a width designated A, said dimension A beingselected as the minimum width at which a vortex strength of 0.25 Mach isobtained when B is of minimum arc sustaining width and C is equal to theinternal diameter of said electrode, and B and C having the relationshipB/C > 0.2 for the transferred mode and > OR = > 4 for thenon-transferred mode; d. gas supply means for introducing an arc gasthrough said electrode into said chamber to produce a vortical flow insaid chamber and nozzle; and e. cooling fluid supply means forintroducing a cooling fluid into said electrode and nozzle to cool thesame.
 2. An apparatus as claimed in claim 1, said nozzle having anannular depression formed in the forward end of said nozzle and in aform effective to prevent laminar flow in the space immediately adjacentthe forward end of said nozzle and surrounding the plasma gnerated bysaid apparatus.
 3. An apparatus as claimed in claim 2 wherein selectedsaid housing sections are adapted with cooling passages to be fluidcooled and including means to provide a cooling fluid to such passages.4. An apparatus as claimed in claim 1 wherein the rearwardmost saidhousing section is electrically grounded.
 5. An apparatus as claimed inclaim 1 wherein selected said housing sections are adapted with coolingpassages to be fluid cooled and including means to provide a coolingfluid to such passages.
 6. An apparatus as claimed in claim 1 whereinsaid forward housing section and nozzle are shaped to provide opposedspaced oppositely curved surfaces between the forward inner end portionof said forward housing section and the oppositely disposed outer nozzlesurface to establish a substantially uniform electrical field betweensaid surfaces and including an insulating refractory material fillingthe space between said surfaces.
 7. An apparatus as claimed in claim 1wherein said fluid cooled electrode includes fluid cooling passagesproximate the forward internal arc generating surface of said electrodeformed in a manner adapted to provide a higher rate of heat removal fromsuch surface than from the more rearward internal non-arc generatingsurface of said electrode.